1. Field of the Invention
The present invention relates in general to an in-plane switching liquid crystal display (IPS-LCD). In particular, the present invention relates to an IPS-LCD with an alignment free structure and a method of using back exposure to form the same.
2. Description of the Related Art
Liquid crystal displays (LCDs) may be classified by the orientation of the LC molecules between the spaced apart glass substrates. In a conventional twisted nematic LCD (TN-LCD), the LC molecules are twisted between the two substrates. In contrast, in an in-plane switching LCD (IPS-LCD), common electrodes and pixel electrodes are formed on a lower glass substrate (TFT substrate) and an in-plane electric field therebetween is generated to rearrange the LC molecules along the electric field. Accordingly, the IPS-LCD has been used or suggested for improving drawbacks of the conventional TN-LCD, such as a very narrow viewing angle and a low contrast ratio.
In order to achieve a better result of the in-plane electric field, a comb-shaped electrode array is built in the IPS-LCD to solve the problems such as an insufficient aperture ratio and crosstalk produced between data lines and common electrodes. FIGS. 1A and 1B are sectional diagrams of a conventional IPS-LCD. FIG. 1A shows the alignment of the LC molecules at an off state, and FIG. 1B shows the alignment of the LC molecules at an on state. The IPS-LCD has a lower glass substrate 10, an upper glass substrate 12, and a liquid crystal layer 14 disposed in a space between the two parallel glass substrates 10 and 12. On the lower glass substrate 10, serving as a TFT substrate, a plurality of strip-shaped common electrodes 16 arranged as a comb-shape structure is patterned on the lower glass substrate 10, an insulating layer 18 is deposited on the common electrodes 16 and the lower glass substrate 10, and a plurality of strip-shaped pixel electrodes 20 arranged as a comb-shape structure is patterned on the insulating layer 18.
As shown in FIG. 1A, before an external voltage is applied to the IPS-LCD, the LC molecules 14A are aligned in a direction parallel to the lower glass substrate 10. As shown in FIG. 1B, when an external voltage is applied to the IPS-LCD, an in-plain electric field 22 is generated between the common electrode 16 and the pixel electrode 20, resulting in a rotation of the LC molecules 14B toward the in-plane electric field 22.
Depending on the material and the structure design of the common electrode 16 and the pixel electrode 20, the conventional comb-shaped electrode array is classified as three types. FIGS. 2A to 2C are sectional diagrams showing three types of the common electrode 16 and the pixel electrode 20 in the conventional comb-shaped electrode array. In the first type, as shown in FIG. 2A, the common electrode 16 and the pixel electrode 20 are patterned on the same plane and made of a transparent conductive material, such as ITO or IZO. In the second type, as shown in FIG. 2B, the common electrode 16 made of a non-transparent conductive material, such as Al and MoW, is patterned on the lower glass substrate 10 followed by depositing the insulating layer 18, and then the pixel electrode 20 made of a transparent conductive material, such as ITO or IZO, is patterned on the insulating layer 18. In the third type, as shown in FIG. 2C, the common electrode 16 and the pixel electrode 20 are patterned on the same plane and made of a non-transparent conductive material, such as Al and MoW.
FIG. 3 is a simulation result of the optical characteristics of opaque electrodes (Al) and transparent electrodes (ITO). The transmittance is estimated 1.25 times when one of the pair electrodes is transparent, and 1.5 times when both of the pair electrodes are transparent respectively. The first type (FIG. 2A) can provide a greater luminance to the IPS-LCD than the second type (FIG. 2B) and the third type (FIG. 2C). The first type (FIG. 2A), however, provides a worsen view-angle characteristic than the second type and the third type. Also, the third type severely decreases the luminance of the IPS-LCD because most of the light is blocked by the non-transparent conductive material. Therefore, the second type (FIG. 2B) is the most common type used in the conventional comb-shaped electrode array.
FIGS. 4A to 4C show the response characteristics of each electrode type corresponding to the first type (FIG. 2A), the second type (FIG. 2B) and the third type (FIG. 2C), respectively. The frame frequency dependence on the flicker property are observed in the second type, but not observed in other types. This means that LC molecules move on transparent electrodes and the behavior influences that optical characteristics. Also, as shown in FIG. 4B, the luminance variation is found in the second type. This phenomenon is caused by flexo-electro polarity when a voltage is applied to the electrodes, resulting in the offset flicker.
FIG. 5A is a top view showing an electrode array within a pixel area of an IPS-LCD according to the prior art, and FIG. 5B is a sectional view along line Ixe2x80x94I of FIG. 5A showing the electrode array of the IPS-LCD according to the prior art. The conventional IPS-LCD 1 has a plurality of pixel areas arranging in a matrix form and constituted by a plurality of gate lines 2 and data lines 4. Each pixel area comprises a TFT structure 6, a comb-shaped common electrode structure 16 and a comb-shaped pixel electrode structure 20. The comb-shaped common electrode structure 16 comprises a bar 16a and three teeth 16b, and the comb-shaped pixel electrode structure 20 comprises a bar 20a and two teeth 20b. By using the second type as shown in FIG. 2B, the teeth 16b and the teeth 20a of different transmittance materials are patterned on different planes.
However, since misalignment in the photolithography process is not easily controlled, it is possible to form different intervals between the common electrodes 16 and the pixel electrodes 20 on the electrode array, resulting in different capacitances and transmittances. FIG. 6 is a simulation result of the misalignment effect. In this practical case, demerits such as trip mura, shot mura and flicker are commonly found in the conventional IPS-LCD.
An object of the present invention is to provide an IPS-LCD with an alignment free structure and a method of using back exposure to form the same. This alignment free structure can solve the demerits of shot mura and flicker found in the prior art.
In each pixel area of the IPS-LCD with an alignment free structure, at least one floating metal layer is disposed between two common electrodes and patterned on the same plane with the common electrodes, and at least one pixel electrode is disposed between the two common electrodes and covers the floating metal layer. The center of the pixel electrode is aligned to the center of the floating metal layer, and each interval between two adjacent common electrode and pixel electrode is fixed at a constant.
A method of forming an in-plane switching liquid crystal display with an alignment free structure, comprises steps of: providing a glass substrate; forming a plurality of gate lines extending in a first direction on the glass substrate; forming a comb-shaped common electrode structure within each predetermined pixel area, wherein the comb-shaped common electrode structure comprises a common bus line parallel to the gate line and at least two common electrodes extending in a second direction that is perpendicular to the first direction; forming a floating metal pattern within each predetermined pixel area, wherein the floating metal pattern comprises at least one floating metal layer extending in the second direction between the two common electrodes; forming an insulating layer to cover the gate lines, the comb-shaped common electrode structure, the floating metal pattern and glass substrate; forming a plurality of data lines extending in the second direction on the insulating layer, wherein the data lines and the gate lines constitute a plurality of pixel areas arranging in a matrix form; forming an passivation layer on the entire surface of the glass substrate; forming a comb-shaped pixel electrode structure disposed in each pixel area on the passivation layer, wherein the comb-shaped pixel electrode structure comprises a bar near the gate line and at least one pixel electrode that extends in the second direction between the two common electrodes and covers the floating metal layer. The center of the pixel electrode is aligned to the center of the floating metal layer, and each interval between two adjacent common electrode and pixel electrode is fixed at a constant.
It is an advantage of the present invention that using back exposure from the back side of the bottom glass substrate to fine tunes the pattern of pixel electrodes. Thus, the center of the pixel electrode can be aligned to the center of the floating metal layer so as to provide the same degree of in-plane electric field in each sub-pixel area. This results in the same capacitance and transmittance in each sub-pixel area to eliminate trip mura, shot mura and flicker found in the conventional IPS-LCD.
This and other objective of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various figures and drawings.